We have reported that extracorporeal immunoadsorption to remove circulating immune complexes (CIC) and IgG results in complete regression of feline lymphosarcoma (LSA) and a reversal of feline leukemia virus infection (FeLV). This was the first report of the therapeutic elimination of an oncoviral infection and also the first time that extracorporeal immunoadsorption has been used to successfully treat LSA. We are intending to investigate the possible mechanisms by which antiviral and antitumor responses may occur after extracorporeal immunoadsorption therapy. We also intend to treat nonviral, feline mammary tumors (FeMT) by extracorporeal immunoadsorption. We have recently demonstrated that intravenous infusion of protein A has an antitumor effect on LSA and plan to study this observation in more detail. In order to remove CIC that contain IgG for study, protein A, which binds nonspecifically to the Fc portion of IgG, will be bound to a solid matrix. This immunoadsorbent will be mixed with the plasma of the treated (FeLV-LSA or FeMT) cats to remove CIC and the treated plasma returned to the host. The CIC can then be eluted from the immunoadsorbent for characterization. To better understand the mechanism of the anti-FeLV and antitumor response, the humoral and cell-mediated immunity of treated cats will be studied. The serum levels of FeLV antigens, FeLV-antibody, and FOCMA antibody will be monitored during the course of treatments. Complement and CIC levels will be measured regularly to detect changes. We will study the biochemical nature of the molecules removed from the plasma of treated animals and quantitate the amounts of FeLV antigen and antibody to FOCMA antibody that may be in the CIC removed by extracorporeal immunoadsorption. The results of these studies will lead to a better understanding of the mechanism by which extracorporeal immunoadsorption eliminates FeLV and induces LSA regression. Our studies may result in the development of better clinical treatments for cancer and certain viral and immune-related diseases. In addition, we may provide basic information about the immunological interactions between the host and its tumor. (IT)